Source to Sink: What makes a free-flowing river?

Securing global freshwater resources for people and a living planet

A free-flowing river runs from its source to its outlet or “sink”—another river, a lake, or the sea—with few obstacles or alterations to how and where it flows. But a river’s connectivity isn’t determined just by water flow within the river channel; other factors matter too, including the river’s capacity to spread laterally into surrounding floodplains, the connection with water in the ground, and when or how much water is flowing over time. Any alterations to these factors—from infrastructure to water regulation—can affect a river’s free-flowing status. To identify and map free-flowing rivers globally, researchers selected five pressure indicators for their considerable potential to impact various aspects of river connectivity.

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Degree of fragmentation

shows, based on a map of the world’s major dams, how much river networks are fragmented by infrastructure like hydropower and irrigation dams.

2

Degree of regulation

determines how strongly particular dams affect a river’s natural flow (the annual average of water flowing downstream), accounting for how much water a dam’s reservoir holds back.

3

Road density

demonstrates where river connectivity is disrupted laterally by roads developed on or through floodplains and culverts.

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Nightlight intensity in urban areas

reveals, through satellite images, where cities and urban infrastructure built through floodplains are likely to affect connectivity or cause river channelization.

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Consumptive water use

measures how much water is removed for irrigation, industry, or municipal uses, and how connectivity is disrupted by these changes.

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